This invention relates to a control system for a lumber kiln and more particularly to maintaining a constant drying rate for the lumber.
Lumber is dried by placing it in a kiln where it is subjected to heat and continuous air flow. Wet bulb and dry bulb temperatures are monitored and steam valves, vents and/or fans are controlled to obtain the desired drying. The difference between the wet bulb and dry bulb temperature is called the "wet bulb depression". The amount of this depression affects the rate of drying of the lumber.
In the typical prior art manually controlled kiln, the moisture content is manually measured. Periodically the operator takes a sample of boards from the load, and an estimate of the moisture content of the boards is obtained by weighing them. From the determination of moisture content the operator determines the appropriate wet bulb and dry bulb set points from a drying schedule, for example the ones supplied by the U.S. Forest Service for different species and thickness of lumber being dried. The operator sets the wet bulb and dry bulb set points on the kiln controller which generates control signals which operate the steam valves and vents to add or subtract heat and/or humidity from the kiln as required to make the kiln conditions match the set points.
Controllers are available for automatically controlling the kiln conditions. For example, the Hildebrandt Company of Germany makes a controller for lumber kilns. In this controller, there is a fixed relationship between moisture content of the lumber and the dry and wet bulb set points. This controller does not produce a constant drying rate.
In U.S. Pat. No. 3,744,144, Weis, the moisture content is monitored by probes which are connected to a sensor which supplies a moisture content signal to the controller 60. The controller 60 functions as a computer in comparing the sensed moisture content signal which a moisture content program such as that set forth in column 4 of the patent. The schedule of column 4 is a typical drying schedule where particular dry bulb and wet bulb temperature set points are specified for particular ranges of moisture content. This system has the disadvantage of changing the set points in steps. The system does not achieve a constant drying rate.
In U.S. Pat. No. 4,176,464, Randolph, the system continuously monitors moisture content by measuring the weight of the load. This load signal is applied to an automatic control circuit 82 which also receives wet and dry bulb signals. In column 6 the patent teaches measuring the rate of change of moisture content and automatically controlling the kiln in response to this measurement. The Randolph patent discloses one system for maintaining a desired rate of change of moisture in the lumber, but it does not recognize that a constant drying rate should be maintained.
Prior art lumber kilns typically have a reversible fan or fans for moving air through the lumber first in one direction and then in the other direction. In U.S. Pat. No. 2,270,815, Vaughn, the fans are reversed when the temperature reaches the point at which continued operation with the same wet bulb depression would cause objectionable checking and cracking. In this way, maximum permissible drying supposedly is achieved.